The immune system has the capacity to protect the host from harmful organisms while maintaining a state of immunological tolerance, or non-reactivity, to self-proteins and innocuous antigens. Disruption of immunological tolerance can result in allergy to common, environmental antigens such as house dust mite (HDM). Although allergy manifests in a variety of conditions, recent attention has focused heavily on asthma due to its growing prevalence in developed and undeveloped nations alike. Unfortunately, pharmacologic therapies for asthma provide only short-term symptom relief without specifically altering the underlying immune response to the instigating allergen and thus have marginal effects on reducing the overall rate and economic burden of asthma. It has therefore become crucial to define the mechanisms that govern the development of immunological tolerance in order to improve upon the standards of care for patients with asthma. Previous studies have added tremendously to our working knowledge of immunological tolerance by establishing the T regulatory cell (Treg) as a key player in tolerance development in animal models of asthma. Studies in human subjects receiving allergen-specific immunotherapy (SIT), a disease-modifying treatment that restores normal immunity in patients with allergy and asthma, have also associated Tregs with development of immunological tolerance (i.e. reduction of symptoms). However, the mechanism by which Tregs are generated and maintained in the settings of asthma and SIT is currently unknown. The type II-activated macrophage (M$-II), a specific macrophage subtype that is suspected of playing a role in immune regulation and tolerance formation, may be a potential candidate for generation of Tregs in the lung compartments through secretion of soluble, anti-inflammatory mediators (cytokines) required for Treg activation. This proposal hypothesizes that regulatory M$s-II drive the induction of immunological tolerance to house dust mite antigen through influences on Treg activity. The overall aims of the proposal center on investigating whether (1) development of M$s-II following chronic inhalational exposure to HDM antigen mediates tolerance formation through stimulatory effects on Tregs (2) tolerance formation following SIT for treatment of HDM- induced asthma results from M$-II-mediated effects on Treg activity. Overall impact: Successful accomplishment of the aforementioned aims will elucidate pulmonary macrophages as a novel, therapeutic target for patients who suffer from asthma.